1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing a lithium secondary cell, and more specifically, it relates to a method of and an apparatus for manufacturing a lithium secondary cell by forming an active material layer on a collector by a method supplying raw material through discharge into a vapor phase.
2. Description of the Background Art
As to a lithium secondary cell recently subjected to flourishing research and development, cell characteristics such as the charge and discharge voltages, the operating cycle life and the storage characteristics remarkably depend on the employed electrodes. Therefore, active materials employed for the electrodes are improved for improving and upgrading the cell characteristics.
It is known that a cell having high energy density per weight and per volume is obtained when employing lithium metal as a negative active material. In this case, lithium is deposited by charging and dissolved by discharging on a negative electrode. When the cell is repetitively charged and discharged, lithium is repetitively deposited and dissolved on the negative electrode. Thus, lithium is disadvantageously dendritically deposited on the negative electrode. Consequently, inner shorting is disadvantageously caused.
To this end, there has been proposed a lithium secondary cell suppressing the aforementioned dendritic deposition of lithium by employing aluminum, silicon or tin electrochemically alloyed with lithium in charging as a negative electrode active material, as reported in Solid State Ionics, 113–115, p. 57 (1998), for example. Among these materials, silicon having particularly large theoretical capacity is a prospective material as the active material for the negative electrode of a cell exhibiting high capacity.
The assignee of the present invention has proposed an electrode for a lithium secondary cell having a microcrystalline silicon layer or an amorphous silicon layer formed on a collector by a method supplying raw material through discharge into a vapor phase such as CVD or sputtering in International Laying-Open No. WO01/29912 as an electrode for a lithium secondary cell employing silicon as a negative electrode active material and exhibiting excellent operating cycle characteristics.
The active material layer is formed on the collector by the aforementioned method supplying raw material through discharge into a vapor phase generally under a vacuum.
In general, however, an electrode preparer for forming the active material layer on the collector and a cell preparation apparatus for finally preparing the cell with the collector (electrode) formed with the active material layer are set on different positions. After forming the active material layer on the collector, therefore, the collector (electrode) formed with the active material layer must be temporarily discharged from the electrode preparer into the atmosphere and thereafter moved to the cell preparation apparatus for preparing the cell. When exposed to the atmosphere when making transition from preparation of the electrode to preparation of the cell; the surface of the active material layer is disadvantageously oxidized and absorbs moisture.
Moisture absorbed by the active material layer of the nonaqueous lithium secondary cell reacts with LiPF6 or the like employed as lithium salt for an electrolyte to form hydrofluoric acid in the electrolyte, disadvantageously resulting in reduction of cell characteristics. When the surface of a negative electrode active material is oxidized, lithium is irreversibly used for reducing the active material. Therefore, the capacity of a positive electrode is reduced to disadvantageously reduce the cell capacity.